EFFECTS OF HUMAN ACTIVITIES ON THE INTERACTION OF ... - USGS

EFFECTS OF HUMAN ACTIVITIES

ON THE INTERACTION OF

GROUND WATER AND SURFACE WATER

Human activities commonly affect the distribution, quantity, and chemical quality of water

resources. The range in human activities that affect

the interaction of ground water and surface water is

broad. The following discussion does not provide

an exhaustive survey of all human effects but

emphasizes those that are relatively widespread. To

provide an indication of the extent to which

humans affect the water resources of virtually all

landscapes, some of the most relevant structures

and features related to human activities are superimposed on various parts of the conceptual landscape (Figure 25).

The effects of human activities on the quantity and quality of water resources are felt over

a wide range of space and time scales. In the

following discussion, ¡°short term¡± implies time

scales from hours to a few weeks or months, and

¡°long term¡± may range from years to decades.

¡°Local scale¡± implies distances from a few

feet to a few thousand feet and areas as large as a

few square miles, and ¡°subregional and regional

scales¡± range from tens to thousands of square

miles. The terms point source and nonpoint source

with respect to discussions of contamination are

used often; therefore, a brief discussion of the

meaning of these terms is presented in Box M.

Agricultural Development

Agriculture has been the cause of significant

modification of landscapes throughout the world.

Tillage of land changes the infiltration and runoff

characteristics of the land surface, which affects

recharge to ground water, delivery of water and

sediment to surface-water bodies, and evapotranspiration. All of these processes either directly or

indirectly affect the interaction of ground water and

surface water. Agriculturalists are aware of the

substantial negative effects of agriculture on water

resources and have developed methods to alleviate

some of these effects. For example, tillage practices have been modified to maximize retention of

water in soils and to minimize erosion of soil from

the land into surface-water bodies. Two activities

related to agriculture that are particularly relevant

to the interaction of ground water and surface

water are irrigation and application of chemicals to

cropland.

54

Figure 25. Human activities and structures, as depicted

by the distribution of various examples in the conceptual landscape, affect the interaction of ground water

and surface water in all types of landscapes.

55

M

Point and Nonpoint

Sources of Contaminants

Contaminants may be present in water or in air as

a result of natural processes or through mechanisms of

displacement and dispersal related to human activities.

Contaminants from point sources discharge either into ground

water or surface water through an area that is small relative to

the area or volume of the receiving water body. Examples of

point sources include discharge from sewage-treatment

plants, leakage from gasoline storage tanks, and seepage

from landfills (Figure M¨C1).

Nonpoint sources of contaminants introduce

contaminants to the environment across areas that are

large compared to point sources, or nonpoint sources may

consist of multiple, closely spaced point sources. A nonpoint

source of contamination that can be present anywhere, and

affect large areas, is deposition from the atmosphere, both

by precipitation (wet deposition) or by dry fallout (dry deposition). Agricultural fields, in aggregate, represent large areas

through which fertilizers and pesticides can be released to the

environment.

The differentiation between point and nonpoint sources

of contamination is arbitrary to some extent and may depend

in part on the scale at which a problem is considered. For

example, emissions from a single smokestack is a point

source, but these emissions may be meaningless in a regional

analysis of air pollution. However, a fairly even distribution of

tens or hundreds of smokestacks might be considered as a

nonpoint source. As another example, houses in suburban

areas that do not have a combined sewer system have individual septic tanks. At the local scale, each septic tank may

be considered as point source of contamination to shallow

ground water. At the regional scale, however, the combined

contamination of ground water from all the septic tanks in

a suburban area may be considered a nonpoint source of

contamination to a surface-water body.

Figure M¨C1. The transport of contamination from a point

source by ground water can cause contamination of surface

water, as well as extensive contamination of ground water.

Waste site

of

ion

ect

flow

Dir

ter

-wa

und

gro

er

Riv

Contaminant

plume

56

IRRIGATION SYSTEMS

Surface-water irrigation systems represent

some of the largest integrated engineering works

undertaken by humans. The number of these

systems greatly increased in the western United

States in the late 1840s. In addition to dams on

streams, surface-water irrigation systems include

(1) a complex network of canals of varying size

and carrying capacity that transport water, in many

cases for a considerable distance, from a surfacewater source to individual fields, and (2) a drainage

system to carry away water not used by plants that

may be as extensive and complex as the supply

system. The drainage system may include underground tile drains. Many irrigation systems that

initially used only surface water now also use

ground water. The pumped ground water

commonly is used directly as irrigation water, but

in some cases the water is distributed through the

system of canals.

Average quantities of applied water range

from several inches to 20 or more inches of water

per year, depending on local conditions, over the

entire area of crops. In many irrigated areas, about

75 to 85 percent of the applied water is lost to

evapotranspiration and retained in the crops

(referred to as consumptive use). The remainder of

the water either infiltrates through the soil zone to

recharge ground water or it returns to a local

surface-water body through the drainage system

(referred to as irrigation return flow). The quantity

of irrigation water that recharges ground water

usually is large relative to recharge from precipitation because large irrigation systems commonly are

in regions of low precipitation and low natural

recharge. As a result, this large volume of artificial

recharge can cause the water table to rise (see

Box N), possibly reaching the land surface

in some areas and waterlogging the fields. For this

reason, drainage systems that maintain the level of

the water table below the root zone of the crops,

generally 4 to 5 feet below the land surface, are an

essential component of some irrigation systems.

The permanent rise in the water table that is maintained by continued recharge from irrigation return

flow commonly results in an increased outflow of

shallow ground water to surface-water bodies

downgradient from the irrigated area.

57

N

Effects of Irrigation Development

on the Interaction of

Ground Water and Surface Water

Nebraska ranks second among the States with respect

to the area of irrigated acreage and the quantity of water used

for irrigation. The irrigation water is derived from extensive

supply systems that use both surface water and ground water

(Figure N¨C1). Hydrologic conditions in different parts of

Nebraska provide a number of examples of the broad-scale

effects of irrigation development on the interactions of ground

water and surface water. As would be expected, irrigation

systems based on surface water are always located near

streams. In general, these streams are perennial and (or)

have significant flow for at least part of the year. In contrast,

irrigation systems based on ground water can be located

nearly anywhere that has an adequate ground-water

resource. Areas of significant rise and decline in ground-water

levels due to irrigation systems are shown in Figure N¨C2.

Ground-water levels rise in some areas irrigated with surface

water and decline in some areas irrigated with ground water.

Rises in ground-water levels near streams result in increased

ground-water inflow to gaining streams or decreased flow from

the stream to ground water for losing streams. In some areas,

it is possible that a stream that was losing water before development of irrigation could become a gaining stream following

irrigation. This effect of surface-water irrigation probably

caused the rises in ground-water levels in areas F and G in

south-central Nebraska (Figure N¨C2).

EXPLANATION

Surface-water

irrigation project

0

20

40 MILES

Figure N¨C1. Nebraska is one of the most extensively irrigated States in the Nation. The irrigation water comes from

both ground-water and surface-water sources. Dots are irrigation wells. (Map provided by the University of Nebraska,

Conservation and Survey Division.)

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